Dynamic in vivo measurement of acute lymphoblastic leukaemia interactions within bone marrow microenvironments

Abstract

The bone marrow (BM) contains multiple, distinct microenvironments, or niches, that regulate the function of haematopoietic stem and progenitors cells to ensure a balanced production of all mature blood cells. It has been suggested that blood cancers might be dependent on signals from these niches during development of disease and evasion from chemotherapy. Leukaemia is known to remodel the BM microenvironment, and therefore its interaction with it must be dynamic. However, our current knowledge of leukaemia biology is derived ex vivo from flow cytometric analysis, which therefore lacks information about cell localization within its tissue of origin, and static images, which cannot describe the dynamics of leukaemia interaction with the BM microenvironment. We have developed an intra-vital microscopy system to visualize the interaction of Notch-driven T cell acute lymphoblastic leukaemia (T-ALL) with known components of the BM niche. We were able to collect 3 dimensional data on the distribution of T-ALL cells across entire BM cavities, to time-lapse record the behaviour of identified leukaemia cells and colonies, and to re-image individual mice prior to, and following chemotherapy. Our results reveal that T-ALL cells are not dependent on specific BM microenvironments for propagation of disease or selection of a subpopulation of chemo-resistant clones, instead suggesting that a stochastic mechanism underlies both processes. In contrast, T-ALL leads to rapid remodelling of the endosteal niche in late stages of disease, including a complete loss of mature osteoblastic cells whilst perivascular niches are maintained. These findings provide the first dynamic analysis of leukaemia cell interactions with the bone marrow microenvironment in vivo and imply that cell intrinsic mechanisms, rather than signals from specific niches, are responsible for BM invasion and survival of chemo-resistant T-ALL. These same mechanisms are likely to lead to the extensive microenvironment remodelling observed in late stages of disease. The bone marrow (BM) contains multiple, distinct microenvironments, or niches, that regulate the function of haematopoietic stem and progenitors cells to ensure a balanced production of all mature blood cells. It has been suggested that blood cancers might be dependent on signals from these niches during development of disease and evasion from chemotherapy. Leukaemia is known to remodel the BM microenvironment, and therefore its interaction with it must be dynamic. However, our current knowledge of leukaemia biology is derived ex vivo from flow cytometric analysis, which therefore lacks information about cell localization within its tissue of origin, and static images, which cannot describe the dynamics of leukaemia interaction with the BM microenvironment. We have developed an intra-vital microscopy system to visualize the interaction of Notch-driven T cell acute lymphoblastic leukaemia (T-ALL) with known components of the BM niche. We were able to collect 3 dimensional data on the distribution of T-ALL cells across entire BM cavities, to time-lapse record the behaviour of identified leukaemia cells and colonies, and to re-image individual mice prior to, and following chemotherapy. Our results reveal that T-ALL cells are not dependent on specific BM microenvironments for propagation of disease or selection of a subpopulation of chemo-resistant clones, instead suggesting that a stochastic mechanism underlies both processes. In contrast, T-ALL leads to rapid remodelling of the endosteal niche in late stages of disease, including a complete loss of mature osteoblastic cells whilst perivascular niches are maintained. These findings provide the first dynamic analysis of leukaemia cell interactions with the bone marrow microenvironment in vivo and imply that cell intrinsic mechanisms, rather than signals from specific niches, are responsible for BM invasion and survival of chemo-resistant T-ALL. These same mechanisms are likely to lead to the extensive microenvironment remodelling observed in late stages of disease.